In treating disease, it is important to detect morphological and functional changes caused by the disease in the living body at an early stage of the disease. Especially for treatment of a cancer, to know the site and size of the tumor beforehand is an extremely important means to determine strategies and protocols for future treatment. Methods so far applied include biopsy by puncture and the like, as well as imaging diagnosis such as X-ray imaging, MRI, ultrasound imaging and the like. Biopsy is an effective means for definitive diagnosis, however, it places great burden on a patient to be diagnosed, and also is not suitable for tracing changes with time in lesions. X-ray imaging and MRI inevitably cause exposure of a patient to be diagnosed with irradiation or electromagnetic wave. In addition, conventional imaging diagnoses as mentioned above require complicated operation and a prolonged time for measurement and diagnosis. A large size of an apparatus also makes it difficult to apply these methods during surgical operation.
One of reported image diagnoses includes fluorescence imaging (Lipspn R. L. et al., J. Natl. Cancer Inst., 26, 1-11 (1961)). This method employs a substance as a contrast agent that emits fluorescence upon exposure to an excitation light having a specific wavelength. The method comprises the step of exposing a body with an excitation light from outside the body and then detecting fluorescence emitted from the fluorescent contrast agent in vivo.
An example of the fluorescent contrast agent include, for example, a porphyrin compound that accumulates in tumor and is used for photodynamic therapy (PDT), e.g., haematoporphyrin. Other examples include photophyrin and benzoporphyrin (see, Lipspn R. L. et al., supra, Meng T. S. et al., SPIE, 1641, 90-98 (1992), WO84/04665 an the like). However, these compounds have phototoxicity since they are originally used for PDT (PDT requires such property), and accordingly, these compounds are not desirable as diagnostic agents.
Retinal circulatory microangiography using a known fluorescent dye, such as fluorescein, fluorescamine, and riboflabin, has been known (U.S. Pat. No. 4,945,239). However, these fluorescent dyes emit fluorescence in a region of a visible light of 400-600 nm which only achieves low transmission through living tissue, and consequently, detection of a lesion in a deeper part of a body is almost impossible.
Cyanine compounds including indocyanine green (hereinafter abbreviated as “ICG”), which are used to determine liver function and cardiac output, have been also reported to be useful as fluorescent contrast agents (Haglund M. M. et al., Neurosurgery, 35, 930 (1994), Li, X. et al., SPIE, 2389, 789-797 (1995)). Cyanine compounds have absorbance in a near infrared light region (700 to 1300 nm).
Near infrared light has a high transmission property through living tissues and can pass through the skull of about 10 cm, and from these reasons, said light has been focused recently in the field of clinical medicine. For example, the optical CT technique (a CT technique using optical transmission of a medium) has become focused as a new technology in the clinical field, because near infrared light can pass through a living body and, oxygen concentration and circulation in vivo can be detected by using a light within this region.
The cyanine compound emits fluorescence in the near infrared region, a light of which region has excellent permeability in living tissues as explained above, and accordingly a use as a fluorescent contrast agent has been proposed. Various cyanine compounds have been developed in recent years, and approaches for use as fluorescent contrast agents have been made (WO96/17628, WP97/13490 and the like). However, an agent having a satisfactory distinguishing ability of a lesion from normal tissues, i.e., an agent having a satisfactory selectively to a target site to be imaged, has not yet been available.